A catalytically active oscillator made from small organic molecules

Oscillatory systems regulate many biological processes, including key cellular functions such as metabolism and cell division, as well as larger-scale processes such as circadian rhythm and heartbeat1–4. Abiotic chemical oscillations, discovered originally in inorganic systems5,6, inspired the development of various synthetic oscillators for application as autonomous time-keeping systems in analytical chemistry, materials chemistry and the biomedical field7–17. Expanding their role beyond that of a pacemaker by having synthetic chemical oscillators periodically drive a secondary function would turn them into significantly more powerful tools. However, this is not trivial because the participation of components of the oscillator in the secondary function might jeopardize its time-keeping ability. We now report a small molecule oscillator that can catalyse an independent chemical reaction in situ without impairing its oscillating properties. In a flow system, the concentration of the catalytically active product of the oscillator shows sustained oscillations and the catalysed reaction is accelerated only during concentration peaks. Augmentation of synthetic oscillators with periodic catalytic action allows the construction of complex systems that, in the future, may benefit applications in automated synthesis, systems and polymerization chemistry and periodic drug delivery.


Materials and Methods Supplementary Text
Figs. S1 to S60 Tables S1 to S13 Schemes S1 to S22

General Information
Purification of the products was performed by flash chromatography using a BioTage Selekt® flash chromatography system and Claricep Flash irregular silica 40-60 μm columns. NMR spectroscopic data was collected on a Varian MercuryPlus ( 1 H at 400 MHz; 13 C at 101 MHz; 31 P at 162 MHz ) equipped with a 400 Autosw probe, a Varian 400MR ( 1 H at 400 MHz; 13 C at 101 MHz; 31 P at 162 MHz ) equipped with a OneNMR probe, a Varian Inova 500 ( 1 H at 500 MHz, 13 C at 126 MHz, 31 P at 202 MHz) equipped with a Varian 5 mm PFG SW probe and a Bruker NEO ( 1 H at 600 MHz; 13 C at 151 MHz; 31 P at 243 MHz; NOESY at 600 MHz) equipped with a SmartProbe BBFO. Chemical shifts are reported in parts per million (ppm) relative to residual solvent peak (CDCl 3 , 1 H: 7.26 ppm; 13 C: 77.16 ppm). Coupling constants are reported in Hertz (Hz). Multiplicity is reported with the usual abbreviations (s: singlet, d: doublet, dd: doublet of doublets, t: triplet, q: quadruplet, m: multiplet).
Exact mass spectra were recorded on a LTQ Orbitrap XL apparatus with ESI ionization.
In situ FTIR reaction analysis was performed using a Mettler-Toledo ReactIR TM 700 instrument fitted with a DiComp (diamond) probe and a AgX Fiber Conduit and a liquid N 2 cooled MCT detector. Spectra were recorded with 8 cm -1 resolution. Analysis of IR spectra were analyzed using iC IR 7.1, calibration lines were made using iC Quant.
Flow experiments were performed using a dual channel syringe pump (New Era SyringeTWO) for infusion and a single channel syringe pump for extraction (New Era SyringeOne).
Unless otherwise indicated, reagents and substrates were purchased from commercial sources and used as received. Solvents not required to be dry were purchased as technical grade and used as received.
Modelling and data analysis was done in R. 1 Data analysis was performed using the tidyverse package. 2 Modeling was done using the deSolve package. 3 Parallelization was done using the Foreach 4 package. Scripts used for modelling are included as Supplementary Data (S1-S2) UV measurements were performed with an Analytik Jena Specord S600 spectrometer and analyzed using SpectraGryph 5 .

Synthesis
Scheme S1. Synthesis of Fmoc-piperidine (2) Fmoc-piperidine (2) was synthesized using a modified literature procedure. 6 Fmoc-NHS (7.4 g, 22 mmol, 1.1 eq.) was dissolved in DCM (80 ml) in a 100 ml three-necked flask. The solution was cooled to 0 °C using an ice bath. Piperidine (1, 2.0 mL, 20 mmol, 1.0 eq.) was added over 20 minutes using a syringe pump. The ice bath was removed and the reaction was stirred at room temperature. After 2 hours the reaction was quenched by the addition of acetyl chloride (1.4 mL, 20 mmol, 1.0 eq.). After 30 minutes the reaction mixture was washed once each with water and brine. Organics were dried over MgSO 4 and volatiles were removed in vacuo. The crude product was dissolved in a minimal amount of DCM and purified using flash chromatography (15% EtOAc in pentane 3 CV, 15-30% EtOAc in pentane 10 CV, 30% EtOAc in pentane 3 CV). Fmocpiperidine (4.4 g, 14 mol, 72%) was obtained as a white solid. Spectroscopic data agreed with reported data. 6 Fmoc-piperidine was stored in a freezer to prevent degradation.   (6) Dibenzofulvene (DBF, 6) was synthesized using a modified literature procedure. 7 Fmoc-Cl (1.0 g, 3.9 mmol) was dissolved in methanol (40 ml). Water (10 ml) and potassium hydroxide (0.43 g, 7.7 mmol) were added. The solution was heated at reflux for 1.5 hours. The reaction was then cooled quickly to room temperature with an ice/water bath. The mixture was thrice extracted with pentane (100 ml). Combined organics were dried over MgSO 4 . The crude product was then loaded on celite and volatiles were removed in vacuo (T Bath = 30 °C, P = 750 mbar)the flask was protected from light by covering it in aluminum foil. Pure DBF was obtained by flash chromatography using isocratic elution with pentane. The synthesized DBF was used directly to make a calibration line for the deprotection of Fmoc-piperidine, because pure DBF rapidly polymerizes. Spectroscopic data agreed with reported data.

NMR pulse in batch General analysis procedure
Pulse in batch experiments were followed with 1 H-NMR spectroscopy. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function.
To ensure quantitative concentration data could be obtained from the 1 H-NMR spectra the T1 relaxation times of the followed peaks were determined using an inversion recovery experiment. To ensure quantitative integration, and thus quantitative concentrations, the relaxation time used for obtaining the spectrum should be at least seven times the relaxation time of the slowest relaxing signal. Therefore all 1 H-NMR experiments were carried out with a relaxation time of 40 seconds.
In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized Nmethylpiperidine (5) solution (3 μL) was added. A perforated cap was placed on the tube to ensure that CO 2 (11) can escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1 hour to ensure quantitative integration. For results see Fig. S2 and Fig. S3.
In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized N-methylpiperidine (5) solution (3 μL) was added. A perforated cap was placed on the tube to ensure that CO 2 (11) can escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1 hour to ensure quantitative integration. For results see Fig. S3.
In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized DBU (13) solution (6 μL) was added. A perforated cap was placed on the tube to ensure that CO 2 (11) can escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1.5 hours to ensure quantitative integration. For results see Fig. S2 and Fig. S3.
In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized DBU (13) solution (6 μL) was added. A perforated cap was placed on the tube to ensure that CO 2 (11) can escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1.5 hours to ensure quantitative integration. For results see Fig. S3.

Screening of batch conditions
We started our study with an investigation of the rate laws of the four different reactionsinitiation, autocatalysis, fast inhibition, and slow inhibitionat room temperature using Raman spectroscopy. While determining a rate law for the autocatalysis we found that the decarboxylation of the intermediate carbamic acid to form piperidine (1) and CO 2 (11) was relevant to the kinetics. The rate-law we had determined severely overestimated the rate of production of piperidine (1). The reason for this mismatch is most likely the stability of the carbamic acid in DMSO. 11 We made several attempts to remedy this by either measuring the rate of decarboxylation, or estimating the rate through modelling, but none of these were able to describe the system to our satisfaction.  Rather than continuing this fruitless endeavor we decided to instead investigate whether we could overcome this problem by raising the temperature, thereby ensuring rapid decarboxylation of the carbamic acid. We used the rate laws determined at room temperature to predict what concentrations were necessary for a single oscillation in batch and carried out that experiment a t 60 °C. We then changed the concentrations experimentally until a pulse in batch was achieved.
We defined a successful pulse as having an observable lag phasei.e. several data pointsand returning back to baseline within the experiment time (90 minutes). The screening of conditions for a single oscillation under batch conditions was done using 1 H-NMR spectroscopy. Therefore, the concentration of Fmoc-piperidine (2) was set to a value appropriate for this technique (100 mM). An overview of the used conditions are shown in table S1. The first experiment was carried out with 5 mM p-nitrophenyl acetate (3), 500 mM phenyl acetate (4), and 1 mM DBU (13) (Fig. S2b). We observed a clear lag phase. Additionally, strong positive feedback was observed indicating that decarboxylation now occurs rapidly. However, the concentration of piperidine (1) did not return to baseline quick enough. We carried out a second experiment with the same conditions as the first experiment but a higher loading of phenyl acetate (4) (1500 mM). In this case, a promising pulse was observed (Fig. S2c). There is a clear lag phase, followed by exponential growth, and lastly decay of the concentration of piperidine (1) to the baseline within the experiment time. We further fine-tuned these concentrations in a third experiment to 5 mM p-nitrophenyl acetate (3), 1000 mM phenyl acetate (4), and 0.5 mM DBU (13) (Fig. S2d). We then noticed that the initiation reaction, DBU (13) catalyzed deprotection of Fmoc-piperidine (2) was dependent on the loading of p-nitrophenyl acetate (Fig. S3a). To prevent this from posing a problem to our oscillator we switched to using N-methylpiperidine (5) as our initiator. This base is a much weaker base and so a higher loading had to be used. A pulse in batch was carried out using 5 mM p-nitrophenyl acetate (3), 1000 mM phenyl acetate (4), and 5 mM Nmethylpiperidine (5) (Fig. S2e).

Fig. S3. The influence of p-nitrophenol (12) on pulse in batch experiments with (a) DBU (13) and (b) N-methylpiperidine (5).
Pulse in batch carried out at 60 °C using 100 mM Fmocpiperidine (2), 5 mM p-nitrophenyl acetate (3), 1.0 M phenyl acetate (4) and (a) 0.5 mM DBU (13) or (b) 5 mM N-methylpiperidine (5) in DMSO-d 6 in the presence (Red) and absence (Blue) of 50 mol% p-nitrophenol (12). The experiments were carried out in duplo and the results for all experiments are plotted hence there are two red and two blue traces in a and b.
When DBU (13) is the trigger, we found that the rate of initiation is dependent on the concentration of p-nitrophenyl acetate (3). The p-nitrophenyl acetate (3) itself cannot react with DBU (13), as DBU (13) is a tertiary base and cannot form a stable amide. The leaving group, p-nitrophenol (12), which is released upon acetylation of piperidine (1) is mildly acidic in DMSO. It could be that there is partial proton transfer from p-nitrophenol (12) (pK a 10.8) 12 to the more basic DBU (13) (pK a 13.9) 13 under the reaction conditions. Partial protonation of DBU (13) would mean that the initiation reaction is slowed down, but does still occur. We performed pulse in batch experiments in the presence (Fig. S3, red traces) and absence (Fig. S3, blue traces) of 50 mol% of p-nitrophenol (12) to investigate this effect. We saw that the initiation by DBU is strongly affected by the presence of p-nitrophenol (12) (Fig. S3a). DBU (13) catalyzed Fmoc deprotection is inhibited by the presence of p-nitrophenol (12). With N-methylpiperidine (5) (pK a 10.08) 14 no inhibition by pnitrophenol (12) occurs (Fig. S3b).
Reagent solution (2 ml) was transferred to a quartz cuvette. Bromothymol blue solution (10 μL) was added. The solution was heated to 60 °C and allowed to equilibrate for 10 minutes. Nmethylpiperidine (5) solution (10 μL) was added. A UV/Vis spectrum (500 -1020 nm) was recorded every 30 seconds for 2 hours. Spectra were analyzed using Spectragryph 5 . A simple baseline correction was applied and absorbance at 637 nm was used as a measure of piperidine (2) concentration. For results see Fig. S5. To experimentally confirm the changes in the concentration of piperidine (1) over the course of the oscillation we performed a pulse in batch in the presence of the pH indicator Bromothymol Blue and followed the reaction with UV-Vis spectroscopy ( max = 637 nm). Release of piperidine will lower the pH of the solution and this will be indicated by Bromothymol Blue.
The absorbance of the pH indicator is directly correlated to the concentration of the piperidine ( 1).
We observed a clear peak in the absorbance (Fig. S5) which indicates a rise and fall in the pH of the solution demonstrating the formation of piperidine in the pulse in batch. These solutions were heated to 60 °C and allowed to equilibrate for 10 minutes after which in situ IR spectra were recorded. Using the acquired spectra, a calibration line (Fig. S7). was made using the iC Quant Univariate Calibration software. The concentration of Fmoc-piperidine (2) was followed using peak area analysis of the band between 1167 and 1127 cm -1 with a two-point baseline between 1172 and 1120 cm -1 .

Sample [Fmoc-piperidine (2)] (mM)
Fmoc-piperidine ( (2) in the presence of DBF (6) using in situ IR spectroscopy with peak area analysis of the band between 1167 and 1127 cm -1 with a two-point baseline between 1172 and 1120 cm -1 .

Fig. S7. Calibration line for the concentration of Fmoc-piperidine (2) in the presence of DBF (6) in DMSO at 60 °C using in situ IR spectroscopy with peak area analysis of the band between 1167 and 1127 cm -1 with a two-point baseline between 1172 and 1120 cm -1 .
A stock solution was made of Fmoc-piperidine (2) (768.5 mg, 2.5 mmol) and p-nitrophenyl acetate (3) (4.5 mg, 0.025 mmol) in DMSO (25 ml) using volumetric glassware. Fmoc-piperidine (2) is not stable in solution by itself so 1 mol% of p-nitrophenyl acetate (3) was added as a stabilizer. A stock stolution was made of piperidine (1) (99 μL, 1.0 mmol) in DMSO (1 ml) using volumetric glassware. Mixtures of Fmoc-piperidine (2) stock and DMSO were made in duplo according the table S3. These solutions were heated to 60 °C and allowed to equilibrate for 10 minutes. Piperidine (1) was added according to table S3 and the reaction was followed using in situ IR spectroscopy. The univariate calibration model was used to determine concentrations in the iC IR software package.
Further analysis was done in R 1 using the tidyverse 2 package suite. Initial rates were determined for each experiment. The starting concentrations of Fmoc-piperidine (2) and piperidine (1) were both varied. For both reagents the experimentally determined initial rates were plotted against starting concentration and the observed, pseudo-first order rate constant was then determined by taking the slope of a straight line plotted through these points (Fig. S8). The good fit of the straight line to the initial rates for both reagents proves that the reaction is 1 st order with respect to both reagents. The second order rate constant was determined by dividing the observed rate constant by the concentration of the component that was not varied. The average of these two determined rate constants was taken as the final second order rate constant for the autocatalytic deprotection of Fmoc-piperidine (Equation S1).

Equation S1
-Rate law for the autocatalytic piperidine (1) catalyzed deprotection of Fmocpiperidine (2). The rate constant is the mean of the two values from Fig. S8, the error is the standard deviation propagated from the fits of Fig. S8.
Fmoc-piperidine (2) solution (1 ml) was heated to 60 °C and allowed to equilibrate for 10 minutes. N-methylpiperidine (5) stock was added according to table S4 and the reaction was followed using in situ IR spectroscopy. The Fmoc-piperidine (2) univariate calibration model made in the previous section ( Fig. S7) was used to determine concentrations in the iC IR software.  Table S4. Kinetic runs for the N-methylpiperidine (5) catalyzed deprotection of Fmoc-piperidine (2). Initial rates are the mean of two experiments, errors are standard deviations Further analysis was done in the software in R 1 using the tidyverse 2 package suite. Initial rates were determined for each experiment. The observed rate constant was then determined from the slope between observed initial rate and N-methylpiperidine (5) concentration (Fig. S9). The good fit of the straight line proves that the reaction is 1 st order in N-methylpiperidine (5). Since for the autocatalysis the reaction was first order in Fmoc-piperidine (2) it is assumed that is also the case here. The second order rate constant for the trigger reaction was determined by dividing the observed rate constant by the concentration of Fmoc-piperidine (2) (Equation S2).

Scheme S12. Slow inhibition. Acetylation of piperidine (1) by phenyl acetate (4)
A stock solution was made of 1 M phenyl acetate (4) in DMSO using volumetric glassware. A stock solution was made of 1 M phenol (14) and 1 M PipAc (7) using volumetric glassware. Mixtures of these stock solutions were made according to table S5. These solutions were heated to 60 °C and allowed to equilibrate for 10 minutes after which in situ IR spectra were recorded. Using the acquired spectra a calibration line (Fig. S11) was made using iC Quant Univariate Calibration. The concentration of phenyl acetate (4) was followed using peak height analysis of the band between 1764 and 1753 cm -1 with a two point baseline between 1813 and 1697 cm -1 .

Fig. S10. Representative in situ IR spectra of the acetylation of piperidine (1) and phenyl acetate (4) spectra showing which peak was used to follow the concentration of phenyl acetate (4). The reaction was carried out at 60 °C using 1 M piperidine (1) and 1 M phenyl acetate (4) in DMSO.
The reaction was monitored for 90 minutes.

Sample [PhOAc (4)] (M)
[PipAc  Table S5. (14) in DMSO at 60 °C using in situ IR spectroscopy with peak height analysis of the band between 1764 and 1753 cm -1 with a two point baseline between 1813 and 1697 cm -1 . (14) in DMSO at 60 °C using in situ IR spectroscopy with peak height analysis of the band between 1764 and 1753 cm -1 with a two point baseline between 1813 and 1697 cm -1 .

Fig. S11. Calibration line of the concentration of phenyl acetate (4) in the presence of N-acetyl piperidine (7) and phenol
A stock solution was made of 1 M phenyl acetate (4) in DMSO using volumetric glassware. Mixtures of phenyl acetate stock and DMSO were made in duplo according the table S5. These solutions were heated to 60 °C and allowed to equilibrate for 10 minutes. Piperidine (1) was added according to table S6 and the reaction was followed using in situ IR spectroscopy. The univariate calibration model (Fig. S11) was used to determine concentrations in the iC IR software.  Table S6. Kinetic runs of the acetylation of piperidine (1) and phenyl acetate (4). Initial rates are the mean of two experiments, errors are standard deviations Further analysis was done in R 1 using the tidyverse 2 package suite. Initial rates were determined for each experiment. The starting concentrations of PhOAc (4) and piperidine (1) were both varied. For both reagents the determined initial rates were plotted against starting concentration and the observed rate constant was then determined by taking the slope of a straight line plotted through these points (Fig. S12). The good fit of the straight line to the initial rates for both reagents proves that the reaction is 1 st order in both reagents. The second order rate constant was then determined by dividing the observed rate constant by the concentration of the component that was not varied. The average of these two determined rate constants was then taken as the final second order rate constant for the acetylation of piperidine (1) by phenyl acetate (4) (Equation S3).

Scheme S13. Fast inhibition. Acetylation of piperidine (1) by p-nitrophenyl acetate (3).
Stock solutions were made of p-nitrophenyl acetate (3) (18.1 mg, 0.10 mmol) in DMSO (1 ml), pnitrophenol (12) (13.9 mg, 0.10 mmol) in DMSO (1 ml). Both solutions were diluted 100x and subsequently 10x to arrive at 100 μM solutions. A 2 M piperidine (1) stock solution was made by dissolving piperidine (1) (198 μL, 2.0 mmol) in DMSO (1 ml). To the nitrophenol (12) stock solution was added piperidine solution (50 μL -100 eq. of piperidine (1) w.r.t. nitrophenol (12)). All three stock solutions were then diluted 10x to finally arrive at 10 μM solutions. A calibration line was then made of nitrophenol (12) in the presence of 1 mM piperidine (1) according to table S6 (Fig. S14a). Spectra were recorded at 60 °C after 5 minutes of equilibration. Spectra were analyzed using Spectragryph. 5 A simple baseline correction was applied and absorbance at 433 nm was used as a measure of p-nitrophenol (12) concentration.  The rate constant acetylation of piperidine (1) by p-nitrophenyl acetate (3) was determined using pseudo-first order analysis. In a quartz cuvette was placed 10 μM p-nitrophenyl acetate (3) stock solution (2 ml) and allowed to equilibrate to 60 °C for 5 minutes. Then 0.2 M piperidine (1) stock (10 μL) was added. UV-Vis absorbance spectra were recorded every 0.5 seconds for 40 seconds. The experiment was repeated six times. The concentration of p-nitrophenyl acetate (3) was determined by subtracting the concentration of p-nitrophenol (12) at each time point from the final concentration of p-nitrophenol (12). For each individual reaction the observed rate constant was determined by fitting the 1 st order integrated rate equation. The 2 nd order rate constant for the fast inhibition was then determined by taking the average of all six observed rate constants and dividing it by the concentration of piperidine (Equation S4).

Reaction
Rate law k (M -1 s -1 )  Where k tr is the rate constant for the trigger reaction -N-methylpiperidine (5) catalyzed deprotection of Fmoc-piperidine (2), sv = spacevelocity (flowrate / volume), k ac is the rate constant for autocatalysispiperidine (1) catalyzed deprotection of Fmoc-piperidine (2), k inh1 is the rate constant for the fast inhibition reactionacetylation of piperidine (1) by p-nitrophenyl acetate (3) , k inh2 is the rate constant for the slow inhibition reactionacetylation of piperidine (1) by phenyl acetate (4), in subscripts indicate concentrations fed into the reactor.

Scheme 14
Single pulse in batch carried out in a stirred flask followed with in situ IR.
A 1 M solution of N-methylpiperidine (5)  Reagent solution (4 ml) was transferred to a two-necked flask in an oil bath set to 70 °Ca bath temperature of 70 °C is needed to ensure an internal temperature of 60 °C. An in situ IR probe was placed beneath the solution surface. The mixture was stirred until the temperature had stabilized, or for a minimum of 10 minutes. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. To the flask was added N-methylpiperidine (5) solution (20 μL). IR spectra were recorded every ten seconds for 50 minutes. The following bands were used to follow the components over time (Fig. S15): Fmoc-piperidinearea from 1714 to 1676 cm -1 , baseline from 1724 to 1670 cm -1 , PipAcarea from 1664 to 1616 cm -1 , baseline from 1668 to 1613 cm -1 , DBFarea from 796 to 776 cm -1 , baseline from 796 to 776 cm -1 .

Fig. S15. Representative IR spectra of the stirred pulse in batch
showing which bands are followed for, from left to right, Fmoc-piperidine (2), N-acetyl piperidine (7), and DBF (6). IR spectra were recorded every ten seconds for 50 minutes. The shown spectra represent the time period between 19 and 38 minutes.
Samples (100 μL) were taken every 10 minutes, and once more during the exponential growth phase of the oscillations, and quenched in DMSO-d 6 (400 μL) containing 0.025 M TFA. 1 H-NMR spectra of the samples were recorded with a relaxation time of 40 seconds. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. Concentrations were then determined using 1,3,5-trimethoxybenzene as an internal standard. The following peaks were used to follow the concentrations: DBF (6) -6.27 ppm (s, 2H), 1,3,5trimethoxybenzene -6.14 ppm (s, 3H), Fmoc-piperidine (1) -4.41 ppm (d, 2H), PipAc (7) -1.97 ppm (s, 3H).
Further data analysis was done in R 1 using the tidyverse 2 package suite. We estimated the concentration of the components of the oscillator from the IR spectra by taking the first spectrum as 0% conversion, the final obtained spectrum as 100% conversion, and assuming the absorption scales linearly with concentration in between. These assumptions were validated by comparing the obtained concentration with the concentrations determined with 1 H-NMR for the samples. The NMR concentration data aligns closely with the estimated concentrations determined with the ReactIR (Fig. S16). Simulations were made in R 1 with the deSolve 3 package run in parallel using the Foreach 4 package. Sustained oscillations were defined as a system which gave at least two oscillations where the final amplitude of the oscillation was at least 90% of the initial oscillation. Dampened oscillations were defined as a system which gave at least two oscillations where the final amplitude of the oscillation was less than 90% of the initial oscillation. When sustained oscillations were found the amplitude and the period of the oscillations were determined. Results of this exploration and the script are included as Supplementary Data (S1).
Once a suitable area, where concentrations are high enough to follow the oscillations with in situ IR spectroscopy but low enough that all components are soluble, had been found where oscillations could take place -[p-nitrophenyl acetate (3)  The setup for the flow experiments (Fig. S19) was a three-necked flask in an oil bath set to 70 °C, to ensure an internal termperature of 60 °C. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. Three syringes were used. Two 25 ml Hamilton Gastight glass syringes for infusion and one 50 ml Hamilton Gastight syringe for withdrawal. Infusion was done with a NewEra SyringeTwo syringe pump, withdrawal with a NewEra SyringeOne syringe pump. The reaction was followed with in-situ IR spectroscopy. The following bands were used to follow the components over time (Fig. S15): Fmoc-piperidine (2)area from 1714 to 1676 cm -1 , baseline from 1724 to 1670 cm -1 , PipAc (7)area from 1664 to 1616 cm -1 , baseline from 1668 to 1613 cm -1 , DBF (6)area from 796 to 776 cm -1 , baseline from 796 to 776 cm -1 . (2), N-acetyl piperidine (7), and DBF (6). An IR spectrum was recorded every 30 seconds for 20 hours. The shown spectra represent a single oscillation period between 1. 5

and 4.5 hours.
A 10 mM stock solution of N-methylpiperidine was made by dissolving N-methylpiperidine (5) in DMSO (25 ml) using volumetric glassware. A stock solution was made of Fmoc-piperidine, pnitrophenyl acetate and phenyl acetate according to table S9. The stock solutions were loaded into the 25 ml syringes and placed in the syringe pump. A needle with PTFE tubing (ID 0.56 mm) attached was connected to the syringes. The lines were filled with the stock solutions. The reactor was filled at a flow rate of 12 ml/h for 10 minutes to ensure 4 ml of reactor volume. Then the flowrates were lowered to 0.720 ml/h (in, 2 syringes) and 1.440 ml/h (out). An IR spectrum was recorded every 30 seconds for 20 hours (Fig. S20).

Table S9. Conditions used for the flow oscillation experiments
Data analysis was done in R 1 using the tidyverse 2 package suite. The moment that the reactor has been filled and the flowrate takes its final value is defined as t = 0. Absorbances where normalized by taking the difference between the area and the minimum area and dividing that by the difference between the maximum area and the minimum area. This ratio was then multiplied by a normalization factor to account for differences in Fmoc-piperidine (2) starting concentration. The normalization factor was 1 if the starting concentration of Fmoc-piperidine (2) was 100 mM (for experiments 1-4), or as 0.8 if the starting concentration of Fmoc-piperidine (2) was 80 mM (experiment 5), or as 1.2 if the starting concentration of Fmoc-piperidine (2) was 120 mM (experiment 6). For results see Fig. 3.

Pulse coupled catalysis Pulse in batch with Knoevenagel Condensation
Scheme S15. Single pulse with Knoevenagel catalysis Pulse in batch experiments were followed with 1 H-NMR spectroscopy. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function.
To ensure quantitative concentration data could be obtained from the 1 H-NMR spectra the T1 relaxation times of the followed peaks were determined using an inversion recovery experiment. To ensure quantitative integration, and thus quantitative concentrations, the relaxation time used for obtaining the spectrum should be at least seven times the relaxation time of the slowest relaxing signal. Therefore all 1 H-NMR experiments were carried out with a relaxation time of 40 seconds.
Concentrations were determined using 1,3,5-trimethoxybenzene as the internal standard. The following peaks were used to follow the concentrations (  In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized Nmethylpiperidine (5) solution (3 μL) was added to initiate the reaction. A perforated cap was placed on the tube to ensure CO 2 escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1 hour to ensure quantitative integration. (6) and PipAc (7). The curves are an average of two experiments, error bars are standard deviation;
No formation of coumarin (10) is observed in the lag phase (I) but once free piperidine (1) forms in the exponential phase (II) this formation happens rapidly. Comparing the pulse in batch in the presence of the Knoevenagel condensation with the naked pulse in batch (Fig.S22a) we can see that there are no major differences in the progress of the oscillation. The only difference is a slight broadening of the oscillation in the changes in concentration of 1. The fact that the pulse in batch is essentially unchanged from the earlier batch experiment corroborates that there is minimal interference between the Knoevenagel reaction and the oscillator system.

Control experiments Knoevenagel condensation
A 1 M solution of piperidine (1) was made by dissolving piperidine (1) (99 uL, 1.00 mmol) in DMSO (1 ml) using volumetric glassware. A reagent solution was made with according to table S10 in 2 ml of DMSO-d 6 using volumetric glassware.
In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized piperidine ( 1) solution (6 μL) was added to initiate the reaction. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1 hour to ensure quantitative integration.
For results see Fig. S23 and Fig. S38. (  The oscillations are not significantly affected by the presence of the catalytic reaction (Fig. 4) but the inverse interaction is also taking place. We identified that the presence of acidic protons of phenol (14) and p-nitrophenol (12) might affect the Knoevenagel condensation by shifting the keto-enol equilibrium of dimethylmalonate (9). To investigate this, we carried out isolated Knoevenagel condensations in the presence of these phenols. We found that the phenol (14) had no effect on the condensation and that p-nitrophenol (12) had a mild inhibitory effect. (8)

Alternative catalysis
Knoevenagel with benzaldehyde Scheme S16. Knoevenagel catalysis in a single pulse with a different substrate.
Pulse in batch experiments were followed with 1 H-NMR spectroscopy. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized Nmethylpiperidine (5) solution (3 μL) was added to initiate the reaction. A perforated cap was placed on the tube to ensure CO 2 escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 45 seconds for 1 hour to ensure quantitative integration.
Results can be found in Fig. S25.
Pulse in batch experiments were followed with 1 H-NMR spectroscopy. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function.
Concentrations were determined using 1,3,5-trimethoxybenzene as the internal standard. The following peaks were used (Fig. S23)  In duplo: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized Nmethylpiperidine (5) solution (3 μL) was added to initiate the reaction. A perforated cap was placed on the tube to ensure CO 2 escape. 1 H-NMR spectra were recorded with a relaxation time of 40 seconds every 60 seconds for 2 hour to ensure quantitative integration. (6) and PipAc (7). a, Pulse experiment with Knoevenagel catalysis carried out with 100 mM benzaldehyde (17) and 100 mM dimethyl malonate (9). Piperidine (1, green), Benzaldehyde (Red, 17)

Oscillation coupled catalysis
The setup for the flow experiments (Fig. S19) was a three-necked flask in an oil bath set to 70 °C to ensure an internal temperature of 60 °C. Three syringes were used. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. Two 25 ml Hamilton Gastight glass syringes for infusion and one 50 ml Hamilton Gastight syringe for withdrawal. Infusion was done with a NewEra SyringeTwo syringe pump, withdrawal with a NewEra SyringeOne syringe pump. The reaction was followed with in-situ IR spectroscopy.
A 10 mM stock solution of N-methylpiperidine (5) was made by dissolving N-methylpiperidine in DMSO (25 ml) using volumetric glassware. A stock solution was made of Fmoc-piperidine (2), pnitrophenyl acetate (3), phenyl acetate (4), dimethyl malonate (9), salicyl aldehyde (8) according to table S11. The stock solutions were loaded into the 25 ml syringes and placed in the syringe pump. A needle with PTFE tubing (ID 0.56 mm) attached was connected to the syringes. The lines were filled with the stock solutions. The reactor was filled at a flow rate of 12 ml/h for 10 minutes to ensure 4 ml of reactor volume. Then the flowrates were lowered to 0.720 ml/h (in, 2 syringes) and 1.440 ml/h (out). An IR spectrum was recorded every 30 seconds for 24 hours. The following bands were used to follow the components over time (Fig. S26): Salicyl aldehyde (8)area from 1744 to 1728 cm -1 , baseline from 1748 to 1727 cm -1 , 3-(methoxycarbonyl)coumarin (10)area from 1574 to 1551 cm -1 , baseline from 1575 to 1550 cm -1 , DBF (6)area from 803 to 773 cm -1 , baseline from 804 to 772 cm -1 .

Fig. S26. Representative IR spectra of a flow oscillation experiment with Knoevenagel catalysis
showing which bands are followed for, from left to right, salicyl aldehyde (8), 3-(methoxycarbonyl)coumarin (10), and DBF (6). An IR spectrum was recorded every 30 seconds for 24 hours. The shown spectra represent a single oscillation period between 3.75 and 6.0 hours.
Data analysis was done in R 1 using the tidyverse 2 package suite. The moment that the reactor has been filled and the flowrate takes its final value is defined as t = 0. Absorbances where normalized by taking the difference between the area and the minimum area and dividing that by the difference between the maximum area and the minimum area. This ratio was then multiplied by a normalization factor to account for differences in Fmoc-piperidine (2) starting concentration. The normalization factor was 1 if the starting concentration of Fmoc-piperidine (2) was 100 mM (for experiments 1-3), or as 0.8 if the starting concentration of Fmoc-piperidine (2) was 80 mM (experiment 4), or as 1.2 if the starting concentration of Fmoc-piperidine (2) was 120 mM (experiment 5). For results see Fig. 4 and Fig. S27.

Knoevenagel oscillation at different temperatures
A flow oscillation experiment Knoevenagel catalysis was performed in the same way as before using the conditions from Table S11, entry 1. The temperature of the experiment was set according to table S12. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. For results see Fig. 4.

Experiment T Bath (°C) T Solution (°C) 1
60 50 2 a 80 70 Table S12. Temperatures used for flow oscillation experiments. a some data points had to be excluded from the first pulse due to a bubble blocking the detector

Knoevenagel oscillation with perturbation
A flow oscillation experiment Knoevenagel catalysis was performed in the same way as before using the conditions from Table S11, entry 1. After two pulses had completed the temperature of the oil bath was raised to 80 °C to ensure an internal temperature of 70 °C for thirty minutes then the temperature was returned to 70 °C to ensure an internal temperature of 60 °C. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. For results see Fig. 4

Reproducibility of catalytic oscillator
To test the reproducibility of our catalytic oscillator we repeated the 60 °C using 100 mM Fmocpiperidine (2), 30 mM p-nitrophenyl acetate (3), 1.8 M phenyl acetate (4), 5 mM Nmethylpiperidine (5), 200 mM salicyl aldehyde (8), and 200 mM dimethyl malonate (9) in DMSO with a space velocity of 10 -4 s -1 . We were pleased to find that the obtained periods of 2.2±0.2 and 2.4±0.2 hours were within experimental error of each other (Fig. S28b). In both cases we find that the oscillator performs a few dampening pulses before settling into a sustained oscillation. The number of pulses needed to settle into a sustained oscillation was not conversed over repeated experiments, this can be clearly seen from the limit cycle plots (Fig. S28c). The minor changes between experiments are likely caused by noise in the experimental conditions.  (8), and 200 mM dimethyl malonate (9) in DMSO with a space velocity of 10 -4 s -1 . The oscillation was monitored using in situ IR spectroscopy. Sustained oscillations are obtained for DBF (6, red), salicyl aldehyde (8, pink), and 3-(methoxycarbonyl)coumarin (10, orange

Oscillation enhanced selectivity Simulations
To illustrate how oscillations can lead to enhanced selectivity simulations where run using the previously established model with the addition of two Knoevenagel condensations. The Knoevenagel condensation was assumed to be first order in both reagents and the catalyst leading to a 3 rd order rate law (Equation S5). Where k is the rate constant, Product is a Knoevenagel adduct, P is piperidine, Substrate is an aldehyde substrate and DMM is dimethylmalonate.

Equation S5
-Rate law for the piperidine catalyzed Knoevenagel condensation.
Then models were run with two Knoevenagel condensation with one reaction being fast (k = 1 • 10 0 ) and one slow fast (k = 1 • 10 −2 ). It was found that in the presence of oscillations (Fig. S29b) the fast reaction oscillates between 40% and 80% conversion while the slow reacting substrate oscillates between 0.5% and 2.5% conversion. When the simulation is run without phenyl acetate (4) and with 45 mM of 2 so that the final concentration of 1 is similar to the amplitude of the oscillating experiments, and oscillations are not occurring, the fast substrate reaches full conversion and the slow substrate reaches 17% conversion. Theses simulations demonstrate that using chemical oscillations increased selectivity can be achieved. We then ran a series of simulations that gave different amplitudes of 1 to investigate how that would affect the selectivity. We found that, predictably, in the absence of oscillations as the concentration of 1 increases the conversion of Substrate 2 increases and selectivity drops (Fig.  S30, blue dots). Satisfyingly, we found that an increased amplitude of the oscillation does not result in a higher conversion of Substrate 2. As a consequence, the selectivity towards Substrate 1 remains high regardless of the amplitude of the oscillation (Fig. S30, red dots). We defined selectivity as follows: Selectivity for a given set of conditions was determined by taking the average of the selectivities at each time point.

Pulse
Scheme S18. Oscillation enhanced selectivity in a single pulse.
Pulse in batch experiments were followed with 1 H-NMR spectroscopy. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function.
To ensure quantitative concentration data could be obtained from the 1 H-NMR spectra the T1 relaxation times of the followed peaks were determined using an inversion recovery experiment.
To ensure quantitative integration, and thus quantitative concentrations, the relaxation time used for obtaining the spectrum should be at least seven times the relaxation time of the slowest relaxing signal. Therefore all 1 H-NMR experiments were carried out with a relaxation time of 55 seconds. Concentrations were determined using 1,3,5-trimethoxybenzene as the internal standard. The following peaks were used to follow the concentrations (  In duplo for both solutions: Reagent solution (600 μL) was transferred to an NMR tube. The sample was placed in an NMR spectrometer and heated to 60 °C. After the temperature had stabilized N-methylpiperidine (5) solution (3 μL) was added to initiate the reaction. 1 H-NMR spectra were recorded with a relaxation time of 55 seconds every 60 seconds for 1 hour to ensure quantitative integration.

Sustained Oscillations
Amplitude 1 Estimating piperidine amplitude 1 H-NMR spectra of the samples were recorded with a relaxation time of 40 seconds. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. Concentrations were then determined using 1,3,5-trimethoxybenzene as an internal standard. The following peaks were used to follow the concentrations: DBF (6) -6.27 ppm (s, 2H), 1,3,5trimethoxybenzene -6.14 ppm (s, 3H), Fmoc-piperidine (1) -4.41 ppm (d, 2H), PipAc (7) -1.97 ppm (s, 3H). The concentration of piperidine was estimated by subtracting the concentration of PipAc from the concentration of DBF.

Oscillation enhanced selectivity
The setup for the flow experiments (Fig. S19) was a three-necked flask in an oil bath set to 70 °C to ensure an internal temperature of 60 °C. Three syringes were used. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. Two 25 ml Hamilton Gastight glass syringes for infusion and one 50 ml Hamilton Gastight syringe for withdrawal. Infusion was done with a NewEra SyringeTwo syringe pump, withdrawal with a NewEra SyringeOne syringe pump. The reaction was followed with in-situ IR spectroscopy.
1 H-NMR spectra of the samples were recorded with a relaxation time of 55 seconds. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. Concentrations were then determined using 1,3,5-trimethoxybenzene as an internal standard. The following peaks were used to follow the concentrations: salicyl aldehyde (8)  The following bands were used to follow the components over time (Fig. S26): Salicyl aldehyde (8)area from 1744 to 1728 cm -1 , baseline from 1748 to 1727 cm -1 , 3-(methoxycarbonyl)coumarin (10)area from 1574 to 1551 cm -1 , baseline from 1575 to 1550 cm -1 , DBF (6)area from 803 to 773 cm -1 , baseline from 804 to 772 cm -1 . Some data points had to be excluded from the first pulse due to a bubble blocking the detector.
Scheme S20. Oscillation enhanced selectivity in flow with a higher piperidine amplitude A 10 mM stock solution of N-methylpiperidine (5) was made of 10 mM N-methylpiperidine in DMSO (25 ml) using volumetric glassware. A stock solution was made of 240 mM Fmocpiperidine (2), 60 mM p-nitrophenyl acetate (3), 4000 mM phenyl acetate (4), 400 mM dimethyl malonate (9), 100 mM salicyl aldehyde (8), 100 mM p-hydroxybenzaldehyde (15) and 50 mM 1,3,5-trimethoxybenzene in DMSO (25 ml) using volumetric glassware. The stock solutions were loaded into the 25 ml syringes and placed in the syringe pump. A needle with PTFE tubing (ID 0.56 mm) attached was connected to the syringes. The lines were filled with the stock solutions. The reactor was filled at a flow rate of 12 ml/h for 7.5 minutes to ensure 3 ml of reactor volume. Then the flowrates were lowered to 1.080 ml/h (in, 2 syringes) and 2.060 ml/h (out).
Samples (50 μL) were taken every halve hourbringing the total outflow up to 2.160 ml/h effectively -and quenched in 400 μL of DMSO-d 6 containing 25 mM trifluoroacetic acid.
An IR spectrum was recorded every 60 seconds for 10 hours.
Results can be found in Fig. 5, Fig. S34 and Table S13.

Estimating piperidine amplitude
The setup for the flow experiments (Fig. S19) was a three-necked flask in an oil bath set to 70 °C to ensure an internal temperature of 60 °C. Three syringes were used. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. Two 25 ml Hamilton Gastight glass syringes for infusion and one 50 ml Hamilton Gastight syringe for withdrawal. Infusion was done with a NewEra SyringeTwo syringe pump, withdrawal with a NewEra SyringeOne syringe pump. The reaction was followed with in-situ IR spectroscopy.
1 H-NMR spectra of the samples were recorded with a relaxation time of 40 seconds. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. Concentrations were then determined using 1,3,5-trimethoxybenzene as an internal standard. The following peaks were used to follow the concentrations: DBF (6) -6.27 ppm (s, 2H), 1,3,5trimethoxybenzene -6.14 ppm (s, 3H), Fmoc-piperidine (1) -4.41 ppm (d, 2H), PipAc (7) -1.97 ppm (s, 3H). The concentration of piperidine was estimated by subtracting the concentration of PipAc from the concentration of DBF.

Oscillation enhanced selectivity
The setup for the flow experiments (Fig. S19) was a three-necked flask in an oil bath set to 70 °C to ensure an internal temperature of 60 °C. Three syringes were used. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. Two 25 ml Hamilton Gastight glass syringes for infusion and one 50 ml Hamilton Gastight syringe for withdrawal. Infusion was done with a NewEra SyringeTwo syringe pump, withdrawal with a NewEra SyringeOne syringe pump. The reaction was followed with in-situ IR spectroscopy.
1 H-NMR spectra of the samples were recorded with a relaxation time of 55 seconds. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. Concentrations were then determined using 1,3,5-trimethoxybenzene as an internal standard. The salicyl aldehyde (8, pink) to 3-(methoxycarbonyl)coumarin (10, orange) reaches full conversion. The reaction of p-hydroxybenzaldehyde (15, teal) to dimethyl (p-hydroxybenzylidene)malonate (16, brick red) reaches a conversion of 20%.

Low [1] control experiments
The setup for the flow experiments (Fig. S19) was a three-necked flask in an oil bath set to 70 °C to ensure an internal temperature of 60 °C. Three syringes were used. The internal temperature was monitored using the IR thermometer incorporated in the in situ IR probe. Two 25 ml Hamilton Gastight glass syringes for infusion and one 50 ml Hamilton Gastight syringe for withdrawal. Infusion was done with a NewEra SyringeTwo syringe pump, withdrawal with a NewEra SyringeOne syringe pump. The reaction was followed with in-situ IR spectroscopy.
1 H-NMR spectra of the samples were recorded with a relaxation time of 55 seconds. Spectra were analyzed with MestreNova as such: Apodization was set to exponential = 1. Spectra were phased and baseline corrected. If spectrometer artifacts were present they were removed with a 1% drift correction. Integrals were taken of all relevant peaks using the Integral Graph function. Concentrations were then determined using 1,3,5-trimethoxybenzene as an internal standard. The following peaks were used to follow the concentrations: salicyl aldehyde (8) -10.28 ppm (s, 1H), p-hydroxybenzaldehyde (15) -9.25 ppm (s, 1H), dimethyl (p-hydroxybenzylidene)malonate (16) -6.83 (d, 2H).
A stock solution was made of 10/2 mM piperidine (1) in DMSO (25 ml) using volumetric glassware. A stock solution was made of 400 mM dimethyl malonate (9), 100 mM salicyl aldehyde (8), and 100 mM p-hydroxybenzaldehyde (15) and 50 mM 1,3,5-trimethoxybenzene in DMSO (25 ml) using volumetric glassware. The stock solutions were loaded into the 25 ml syringes and placed in the syringe pump. A needle with PTFE tubing (ID 0.56 mm) attached was connected to the syringes. The lines were filled with the stock solutions. The reactor was filled at a flow rate of 12 ml/h for 10 minutes to ensure 4 ml of reactor volume. Then the flowrates were lowered to 0.720 ml/h (in, 2 syringes) and 1.340 ml/h (out).
Samples (50 μL) were taken every halve hourbringing the total outflow up to 1.440 ml/h effectively -and quenched in 360 μL of DMSO-d 6 containing 25 mM trifluoroacetic acid.
An IR spectrum was recorded every 60 seconds for 8 hours.

Results
To asses semi-quantitatively how well the selectivity increases when catalysis is performed in an oscillatory fashion instead of steady state, we determined the mean conversions towards the products 10 and 16. We then took the ratio between these two mean conversions as a measure of how selective the system is for 10 over 16. In oscillatory system with maximum concentration of piperidine 15 mM the conversion of 10 periodically changes between 52 to 87%, while conversion 16 changes between 2-6%, mean ratio 10/16 reached 19:1. In the control steady state flow experiment the conversion of 10 reaches and remains at 95%, while 16 at 18%, mean ration 10/16 reached 5:1. In oscillatory system with maximum concentration of piperidine 22 mM the conversion of 10 periodically changes between 19 to 97%, while conversion 16 changes between 0-3%, mean ratio 10/16 reached 56:1. In the control steady sate flow experiment the conversion of 10 reaches and remains at 97%, while 16 at 18%, mean ratio 10/16 reached 5:1. Selectivity can be obtained by traditional optimization of a system containing only piperidine catalyst (entries 5&6) . If the reaction is part of a more extensive chemical reaction network this may not be possible. In these cases oscillations could be applied to gain selectivity without altering conditions Results can be found in a Fig. 5a,